BM800 LCD Audioimprov Mod, low output

[Jakub W]

Hey Everyone,

I followed the steps of this article to mod a BM800 type mic to an LDC, but the output is low: https://audioimprov.com/AudioImprov/Mics/Entries/2015/12/20_Modding_a_BM-800_Mic.html

I assume, something went wrong with the DC-DC board. It is getting about 8.7 V I connected the + input at the 470 Ohm resistor and the back of the capsule to either C20 or C19, there where it meets the 1.5 M resistor or ground. The output is very low though. I tried to exactly replicate the DC-DC board.

Is there any way to increase the output and should I ground it somehow. When I tried grounding it to Pin 1 of the XLR connector of the board, it gave no output at all.

Will post pictures of my mod later.

 

[Khron]

the back of the capsule to either C20 or C19, there where it meets the 1.5 M resistor or ground.

Why are you measuring THAT voltage?

What reading are you getting between ground, and the R12/C11 node, and then ground and R13/C13 node?

 

[Jakub W]

Why are you measuring THAT voltage?

What reading are you getting between ground, and the R12/C11 node, and then ground and R13/C13 node?

I was measuring, wherever I could. I am getting 8.61 V

 

[Jakub W]

Here are some quick photod

 

[Jakub W]

I knocked off some SMD resistors, so I replaced them with THT, as I don't have enough skills and my soldering iron is rather basic ATM

 

[Khron]

I was measuring, wherever I could.

Well, not all circuit nodes are the same. Which is why i was asking about probing at the nodes i mentioned.

Here are some quick photod

Yeah, i have NO idea what's going on there Might be some solder contact where there shouldn't be, or some cold joint somewhere, or any number of things.

So that circuit is using a 9v zener? That would explain the 8.7v input you're seeing.

What if, instead of taking the oscillator supply from after that 470ohm resistor, you take it from before that?

What LDC capsule are you using, by the way?

 

[Jakub W]

I knocked off some SMD resistors, so I replaced them with THT, as I don't have enough skills and my soldering iron is rather basic ATM

Well, not all circuit nodes are the same. Which is why i was asking about probing at the nodes i mentioned.



Yeah, i have NO idea what's going on there Might be some solder contact where there shouldn't be, or some cold joint somewhere, or any number of things.

So that circuit is using a 9v zener? That would explain the 8.7v input you're seeing.

What if, instead of taking the oscillator supply from after that 470ohm resistor, you take it from before that?

What LDC capsule are you using, by the way?

Will check, the diodes.

The capsule is scavanged from Avantone CK-6 I modded back in the day by replacing the capsule and had to sell off. It is a 32 mm K67 style single-ended capsule.

What if, instead of taking the oscillator supply from after that 470ohm resistor, you take it from before that?

Directly from XLR PIN 2 or from the leg of Q6?

 

[Khron]

What if, instead of taking the oscillator supply from after that 470ohm resistor, you take it from before that?

Directly from XLR PIN 2 or from the leg of Q6?

I said before that resistor, not "before everything else"...

 

[Jakub W]

I said before that resistor, not "before everything else"...

Sorry for that, I'm a noob at this. So apart from the diodes and connecting beforre the resistor. Do I need to ground the converter, and if so, them where?

 

[Jakub W]

Regarding the zener diodes, I requested 1n4148 at a local electronics shop. AFAIK, they can work with up to 100 v

 

[Khron]

Do I need to ground the converter, and if so, them where?

Well, yes. Electrical CIRCUITS are called that, because the electrons need to go in (at least) a circle.

So you're saying you only have ONE wire connected between the BM800 board and the oscillator board, right now?

Regarding the zener diodes, I requested 1n4148 at a local electronics shop.

Those are "normal" diodes, not zeners. The zener i was wondering about was the one already in the original circuit.

 

[ccaudle]

Regarding the zener diodes, I requested 1n4148

That is not a zener diode at all, that is a signal diode.
Khron is referring to this diode in the circuit of the page you mention:


A zener diode maintains close to a constant voltage when reverse biased, so is used as a simple, low-performance voltage regulator.
That diode should have already been in the microphone before modification.
The combination of that 9.1 zener diode and R1 creates an approximately constant 9.1V, and transistor Q6 buffers that as an emitter-follower circuit so that the emitter of Q6 should be always approximately 8.5V. With low load maybe that is the 8.7V you measured.

Do I need to ground the converter

All the circuitry needs to use a common reference, so yes the converter needs to connect to the same ground that the rest of the microphone circuitry uses.
In a schematic the power and ground symbols have special meaning, and any circuit node with that symbol should have an electrical connection to the other nodes using that same symbol. There are some subtleties around how exactly they connect if you want lowest noise, but in basic terms you should be able to connect your ohm-meter between any points using the symbol circled in this picture and there should be a low resistance connection indicated.

 

[Jakub W]

That is not a zener diode at all, that is a signal diode.
Khron is referring to this diode in the circuit of the page you mention:
View attachment 141032

A zener diode maintains close to a constant voltage when reverse biased, so is used as a simple, low-performance voltage regulator.
That diode should have already been in the microphone before modification.
The combination of that 9.1 zener diode and R1 creates an approximately constant 9.1V, and transistor Q6 buffers that as an emitter-follower circuit so that the emitter of Q6 should be always approximately 8.5V. With low load maybe that is the 8.7V you measured.




All the circuitry needs to use a common reference, so yes the converter needs to connect to the same ground that the rest of the microphone circuitry uses.
In a schematic the power and ground symbols have special meaning, and any circuit node with that symbol should have an electrical connection to the other nodes using that same symbol. There are some subtleties around how exactly they connect if you want lowest noise, but in basic terms you should be able to connect your ohm-meter between any points using the symbol circled in this picture and there should be a low resistance connection indicated.

View attachment 141033

Thanks a lot @Khron and @ccaudle. I knew of the commond ground already, but when I tried to ground the converter, there was no voltage. Only when I connect the DC-DC Converter to the - wire of the capsule and the original PCB, I get the capsule to work, so would I need to upgrade this zener?

 

[Khron]

when I tried to ground the converter

WHERE did you try to "ground the converter,", and HOW?

 

[Jakub W]

I rebuilt the converter and here is what I connect to: Orange goes to the leg of the 470 Ohm resistor, White goes to the capsules edge ("-"), I connected all the specified 5 ground points with black wires apart from C19 and 20, which are connected with pins pads filled with solder. Then I tried connecting them to Pin 1 of the XLR connector through the pad at the bottom of the OG PCB. Afterwards I tried with the "-" pin meant for the capsules ground terminal.

The only way I get sound of it, is when the white and orange wires are connected. I also tried before the resists, no change.

The board works with the stock capsule after my 'repairs', but also requires a healthy amount go gain (75%)

 

[Jakub W]

WHERE did you try to "ground the converter,", and HOW?

@Khron @ccaudle sorry for my lack of brain cells, you"ve helped me get to the solution.

1. I needlessly made additional connections between components to ground them. Connecting the ground cable to C19 was enough
2. The 82K resistor caused a significant voltage drop, from 8.5 to 0.04 V. I got rid of it (probably messed up the connection anyway, even if it was in the original signal path). It wasn't"t in the OG schematic of the article anyway, so they must have added it for a minor reason.

The output isn't mind blowing, but usable, and I already ordered most of the components for the Schoeps Alice build, described in Instructables. Also for some reason the noise filter capacitor causes the capsule to be cu off, but It isn't as important for now.